doc: grammify "allows to"

English language grammar does not allow usage
of the word "allows" directly followed by an
infinitive, declaring constructs like "something
allows to do somestuff" un-grammatical.  Often
it is possible to just insert "one" between "allows"
and "to" to make the construct grammatical, but
usually it is better to re-phrase the statement.

This patch tries to fix 4 examples of "allows to"
usage in qemu doc, but does not address comments
in the code with similar constructs.  It also adds
missing "the" in the same line.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
diff --git a/qemu-doc.texi b/qemu-doc.texi
index e6e20eb..88ec9bb 100644
--- a/qemu-doc.texi
+++ b/qemu-doc.texi
@@ -823,7 +823,7 @@
 qemu-nbd --socket=/tmp/my_socket my_disk.qcow2
 @end example
 
-The use of qemu-nbd allows to share a disk between several guests:
+The use of qemu-nbd allows sharing of a disk between several guests:
 @example
 qemu-nbd --socket=/tmp/my_socket --share=2 my_disk.qcow2
 @end example